Of the anaerobic, nonsporulating, gram-negative bacilli, those of the genus Bacteroides are most frequently associated with serious infections in man. Forty clinical isolates of the species Bacteroides fragilis were tested quantitatively against 24 antibiotics by an agar-dilution method under anaerobic conditions. Clindamycin, the most active antibiotic studied, had a median MIC of 0.19 ng/ml. Rifampin, erythromcyin, lincomycin, and chloramphenicol also inhibited all isolates in clinically attainable concentrations. More than half of the strains tested were resistant to tetracycline. Some isolates were susceptible to penicillin G, ampicillin, carbenicillin, and the cephalosporins, and all were resistant to the aminoglycosides, polymyxins, and semisynthetic penicillinase-resistant penicillins. The genus Bacteroides consists of nonsporulating, gram-negative bacilli which are obligate anaerobes. These organisms predominate numerically in the normal bacterial flora of the large bowel and are important components of the normal flora of the oral cavity and the urogenital tract [1]. Although they have long been recognized as significant human pathogens, the literature on infec